1. Technical Field
The present invention relates to instruments and methods for determining properties of a gas and, more particularly, to a transducer apparatus and a related method for substantially simultaneously determining pressure and one other property (or property combination) of a flowing gas of varying pressure and composition.
2. Description of the Prior Art
The heating value of a gaseous substance is of significant interest because it forms one basis for determining the commercial value of that substance as a fuel. Techniques for measuring the quality of gaseous fuels to ascertain the amount of heat available therefrom are already being used in practice for numerous purposes. One particularly novel technique for determining the heating value of a gaseous fuel is described in co-filed U.S. Patent Application entitled "On-Line Combustionless Measurement and/or Regulation of Gaseous Fuels Fed to Gas Consumption Devices," Ser. No. 07/781,598. In addition to determining heating value of a gaseous fuel based upon parameters such as gas density, thermal conductivity, specific heat, molecular weight, viscosity, etc., proper heating value determination normally requires contemporaneous pressure and temperature sensing for correction of the calculated value to standard conditions. Most, if not all, known techniques for determining such gas parameters as pressure and temperature separately measure each desired property. Further, existing sensing devices are often expensive and complex, requiring extensive electronic support equipment and thus warranting only limited use in complex systems where cost is less critical. Fuel gas quality measurement is further complicated by the fact that combustion gases, and particularly natural gases, are typically distributed together notwithstanding separate origin, composition and properties that differ to a greater or lesser extent from each other.
As an example of the most relevant art, it has been known for some time that a vibratory element, such as a quartz crystal, when exposed to a gas will change its frequency of vibration as the gas pressure changes (see, for example, U.S. Pat. No. 4,644,803 and references cited therein). In addition, references exist in the open literature for determining gas density through frequency shift of an oscillator exposed to a test gas (see, e.g., U.S. Pat. No. 4,734,609). However, along with being inoperable in a changing gas pressure and/or changing gas composition environment, most or all of these devices determine only one gas property such as density or pressure.
Thus, a novel, noncomplex approach to the simultaneous measurement of multiple fuel gas properties has clear advantages over the known art, particularly when implemented in a heating value measurement device such as that described in the referenced co-pending application.